In conventional ring-type optical networks, loop-back techniques havebeen employed to recover from a fiber or node failure. The principle of the loop-back techniques is illustrated in FIGS. 1A and 1B. In this example, nodes 1011 to 1014 are connected through two fibers. One of the two fibers is a working fiber 1021 and the other is a protection fiber 1022. In normal state, signal light exists only on the working fiber 1021, and no signal light exists on the protection fiber 1022. Each of the nodes takes out a signal necessary to its node and outputs a signal addressed to the other node. In case of a failure, e.g., when the fiber is cut at a position marked with "x" in FIG. 1B, signals are transmitted through the protection fiber by switching at both its end nodes. Namely, a crescent-shaped loop is formed as shown in FIG. 1B.
Another failure-recovery technique for ring-type optical networks has been, as shown in FIG. 2, suggested in Toba et al., "Demonstration of Optical FDM Based Self-Healing Ring Network Employing Arrayed-Waveguide-Grating ADM Filters and EDFAS", The Twentieth European Conference on Optical Communication, Tu.A.2(1994) In this technique, for a main signal, the output of an optical transmitter 1051 is divided by an optical divider 1041 to simultaneously pass a common signal through both a working fiber 1021 and a protection fiber 1022, then one of the signals is selected by using an electrical switch 1072 provided on the side of optical receivers 1052, 1053. Meanwhile, the inputting and outputting of an optical amplifier 1061 are monitored by intensity-modulating light with a wavelength different from that of the main signal. For the optical amplifier monitoring light, the failure recovery is conducted by a loop-back technique using a loop-back switches 1081, 1082.
However, in the conventional loop-back techniques, there are several problems that: the switching takes a long time since the path needs to be switched after identifying a failure position; the wavelength dependency of the gain of an optical amplifier, such as an Er-doped optical fiber amplifier(EDFA) and a semiconductor optical amplifier is varied since the transmission distance of optical signal is greatly changed; signal deterioration is accelerated due to the accumulation of spontaneous emission optical noise caused by a non-linear effect in fiber or an increase in the number of amplifier stages resulting from the increase in the transmission distance of optical signal; signal deterioration is accelerated due to the accumulation of cross talk caused by an increase of the number of switch stages; the device becomes larger and more expensive since a loop-back switch needs to be provided in the node; and the switching from protection fiber to working fiber is necessary after the failure recovery.
In the technique of Toba et al., the above problems in the loop-back technique are solved as to only main signals. However, in actual failure recovery, failure recovery as to only main signals makes no sense. Namely, the failure recovery of the optical amplifier monitoring light needs to be conducted simultaneously. Because of the loop-back operation conducted for the optical amplifier monitoring light, the time required for the failure recovery must be determined by a failure recovery time for optical amplifier monitoring light. Also, a switch for conducting the loop-back operation foroptical amplifier monitoring light is always necessary. Therefore, the device becomes larger and more expensive. Further, BED the switching from protection fiber to working fiber is necessary after the failure recovery. Still further, even when only a specific node detects a failure, the loop-back operation needs to be conducted. Therefore, signal paths in all the nodes will be changed.